1. Field of the Invention
This invention relates to a wireless telephone system, and more particularly, is suitably applied to a digital cordless telephone system in which plural portable units are registered at a base unit to communicate between the base unit and the portable unit or between the portable units.
2. Description of the Related Art
As this type of a digital cordless telephone system, there is a personal handyphone system (PHS) scheme wherein wireless communication is performed between a base unit connected to a public telephone network with a wire and portable units or between the portable units in accordance with a time division multiple access (TDMA) scheme. In the PHS-scheme digital cordless telephone system, extension communication is performed between the plural portable units registered at the base unit or between the base unit and the portable units, and communication is performed between the portable unit and a telephone connected to a public telephone network, that is, an outside line.
On the TDMA scheme in the PHS-scheme digital cordless telephone system, the time base is divided into 5 [ms] frames. The respective frames are divided into a total of eight time slots, that is, continuous four transmission time slots and continuous four reception time slots. The duration of each time slot is 625 [.mu.s].
In the PHS-scheme digital cordless telephone system, by using one transmission time slot in one frame on the base unit side and using one reception time slot on the portable unit side, digital data which is comprised of physical slots having the stated number of bits is transmitted from the base unit to the portable unit through a certain frequency. Besides, in the PHS-scheme digital cordless telephone system, by using one transmission time slot in one frame on the portable unit side and using one reception time slot on the base unit side, the digital data which is comprised of the physical slots having the stated number of bits is transmitted from the portable unit to the base unit through the same frequency as the aforementioned frequency.
The PHS-scheme digital cordless-phone system causes the portable units which are in a reception waiting state to intermittently receive only control data which is transmitted from the base unit, so that power consumption of the portable units in this state is decreased. That is, the base unit transmits the control data to the respective portable units by using 625 [.mu.s] for each 125 [ms], that is, by using one time slot for each two hundred time slots. At this time, the base unit transmits the control data intermittently using a dedicated frequency (hereinafter, referred to as a control carrier).
On the other hand, the respective portable units in a reception waiting state receive only the control data transmitted using the control carrier, once for each stated period, for instance, once for every one or two seconds.
When a request to connect has occurred, the base unit transmits an individual number of a called-side portable unit by means of the control data, so as to call up the portable unit having the individual number within one or two seconds. After the portable unit has been called up, the base unit discriminates the contents of the connection request, such as a calling or reception to/from the outside line, extension communication with the base unit, and extension communication with another portable unit, and then assigns the portable unit a time slot for transmission/reception of voice data and an arbitrary communication frequency (hereinafter, referred to as a communication carrier) excepting a frequency for a direct talk between portable units.
Thereby, the respective portable units in a reception waiting state are not needed to operate in a mode of always searching, for instance, ten communication carriers for a direct talk between portable units, and to check whether or not a call for extension communication between portable units exists. Therefore, the respective portable units receive only the control data transmitted from the base unit using the control carrier, so that the power consumption in a reception waiting state is decreased considerably in comparison with that of continuous reception and the portable unit communicates with the base unit or with another portable unit via the base unit.
Here, as shown in FIGS. 1A to 1C, the state will be described wherein, after a base unit CS which accommodates two time slots transmits the control data through the control carrier to assign the transmission/reception time slots and the communication carrier of the frequency ft, the base unit relays communication between portable units PS1 and PS2, where T1 to T4 are transmission time slots and R1 to R4 are reception time slots. The base unit which accommodates two time slots comprises only one oscillator for selecting a frequency (wireless channel) in its wireless section, and switches the oscillation frequency so as to generate the communication carrier of the arbitrary frequency.
As shown in FIG. 1B, the portable unit PS1 transmits digital voice data to the base unit CS by using the communication carrier of the frequency ft at the transmission time slot T1 of the period S1. As shown in FIG. 1A, the base unit CS receives the voice data at the reception time slot R1 of the period S1, and then transfers the voice data to the portable unit PS2 at the transmission time slot T3 of the period S7 by using the communication carrier of the frequency ft. As shown in FIG. 1C, the portable unit PS2 receives the voice data at the reception time slot R1 of the period S7, and causes the time slot to be synchronized.
On the other hand, the portable unit PS2 transmits voice data to the base unit CS by using the communication carrier of the frequency ft at the transmission time slot T1 of the period S3. As shown in FIG. 1A, the base unit CS receives the voice data at the reception time slot R3 of the period S3, and then transfers the voice data to the portable unit PS1 at the transmission time slot T1 of the period S5 by using the communication carrier of the frequency ft. As shown in FIG. 1B, the portable unit PS1 receives the voice data at the reception time slot R1 of the period S5, and causes the time slot to be synchronized.
In this manner, the portable units PS1 and PS2 mutually transmit and receive the respective voice data via the base unit so as to cause the time slots to be synchronized, so that communication between the portable units is performed.
However, in the case where the aforementioned digital cordless telephone system has a portable unit PS3 in addition to portable units, there has been such a problem that the base unit CS is not able to transmit, during talking between the portable units PS1 and PS2, the control data to the portable unit PS3 by using the control carrier of frequency that is different from that of the communication carrier ft, before the talking between the portable units PS1 and PS2 is terminated.
As a result, such a trouble has occurred that the other portable unit, for instance, PS3 in a reception waiting state can not receive the control data using the control carrier from the base unit CS during the talk between the portable units SP1 and SP2, and it is put out of the speakable zone. Besides, even if a connection request based on outside line communication, extension communication, a door-phone, etc. has occurred, the base unit CS has not been able to inform the portable unit SP3 in a reception waiting state that the connection request has occurred.
In order to solve these problems, it may be considered that a base unit CS which accommodates three time slots transmits the control data by using the control carrier during the talk between the portable units. That is, as shown in FIG. 2B, the portable unit PS1 transmits voice data to the base unit CS by using the communication carrier of the frequency ft at the transmission time slot T1 of the period S12. As shown in FIG. 2A, the base unit CS receives the voice data at the reception time slot R2 of the period S12, and then transfers the voice data to the portable unit PS2 by using the communication carrier of the frequency ft at the transmission time slot T4 of the period S18. As shown in FIG. 2C, the portable unit PS2 receives the voice data at the reception time slot R1 of the period S18, and causes the time slot to be synchronized.
On the other hand, the portable unit PS2 transmits voice data to the base unit CS by using the communication carrier of the frequency ft at the transmission time slot T1 of the period S14. As shown in FIG. 2A, the base unit CS receives the voice data at the reception time slot R4 of the period S14, and then transfers the voice data to the portable unit PS1 by using the communication carrier of the frequency ft at the transmission time slot T2 of the period S16. As shown in FIG. 2B, the portable unit PS1 receives the voice data at the reception time slot R1 of the period S16.
As shown in FIG. 2A, in addition to relaying the voice data of the portable units PS1 and PS2, the base unit CS transmits control data which is directed to the portable units, so-called down control data, to another portable unit by using a communication carrier of a frequency fc at the transmission time slot T1 of the period S15. Besides, at the reception time slot R1 of the period S11, the base unit CS receives up control data which has been transmitted from the other portable unit to the base unit by using the control carrier of the frequency fc.
In this connection, the control data is transmitted and received once for each 125 [ms] (25 frames) as described above.
In this manner, the base unit CS becomes capable of transmitting/receiving the control data to the portable unit PS3, as well as relaying the voice data of the portable units PS1 and PS2, owing to the use of three time slots. However, in this state, pairs of such time slots are produced that are neighboring and their frequencies differ from each other, such as the reception time slots R1 and R2, the reception time slot R4 and the transmission time slot T1, and the transmission time slots T1 and T2.
However, when it is tried to obtain the frequencies fc and ft by switching the oscillation frequency of a single oscillator for frequency selection, it is hard to ensure waiting time for stabilization of the frequency after switching. Therefore, as shown in FIG. 3, it has been the only way that a wireless section 1 of the aforementioned conventional base unit CS which accommodates three time slots is provided with two frequency-selecting oscillators 2 and 3, and the two oscillators 2 and 3 are switched alternately by a VCO switch 4 so as to obtain the frequencies fc and ft. Therefore, such a problem has existed that the constitution of the wireless section of the base unit CS is complicated.
In this connection, the frequency-selecting oscillators 2 and 3 have been composed into frequency synthesizers, each of which has a phase locked loop (PLL) 5 and a voltage controlled oscillator (VCO) 6.